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 Essay - may/june 2006
Seed ecology III: seeds and weeds
James C. Delouche
Professor Emeritus Mississippi State University
Rice is one of the world's major staple food crops. The ecosystems in which it is cultivated are surely among the most complex and diverse in crop agriculture. There are more than 20 species of Oryza, the rice genus, but only two species are cultivated: Oryza sativa, the Asian and most important species, and O. glaberrima, the African species that began to be replaced by O. sativa with its introduction into Africa about 450 years ago. Interestingly, the Asian and African rices were successfully crossed in the 1990s to produce a "new rice for Africa" termed NERICA, that is said to combine the good traits of the two species. The tremendous variability in rice, especially in O. sativa, has permitted the development of types for cultivation in a greater diversity of ecosystems than any other crop.
Rice is cultivated from humid tropical areas such as Java to temperate areas such as North Korea. It is cultivated in rainfed uplands where there is no flooding, in rainfed lowlands with some periodic flooding, in rainfed and irrigated areas, in irrigated arid areas, in deep water (50 - 100 cm) and as floating rice in very deep water (> 5 m). The ecological diversity of rice is accompanied by a diversity in the methods used to establish the crop.
Weedy Rices
The transplanting into puddled soil system for rice culture was undoubtedly developed and widely adopted thousands of years ago to facilitate the control of weeds. It has, indeed, been very effective in minimizing the establishment of one group of very troublesome weeds, the weedy rices. The weedy rices consist of both conspecific feral types and wild relatives of different Oryza species. The conspecific feral types of O. sativa are usually called "red rices" because most of them produce grain with a red covering or pericarp. They are major weeds in direct seeded, drilled or broadcast rice fields and difficult to control since they are of the same species which precludes the use of selective herbicides.
Red rice ecotypes are essentially varieties or strains of rice that are highly successful as weeds for several reasons. First, they possess most of the features and traits that contribute to the success of weedy plants such as excellent adaptation to the agronomic practices and ecological conditions favored for the crop they infest, a life cycle closely synchronized with the crop, abundant seed production, easy and early seed shattering, seed dormancy, high seed vigor and competitive growth. Second, they possess some special or unique traits that derive from their conspecific relationship with cultivated rice. These include hybridization with cultivated rice resulting in a swarm of segregates from which new, better adapted weedy ecotypes are naturally selected, seeds and juvenile plants that are difficult to distinguish from those of cultivated rice, and the red pericarp that extends losses from reduced yield in the field to reduced quality and discounts at the mill. These shared features and unique traits of the red rices permit them to exploit the varied ecology of drill or broadcast seeded, rainfed and/or irrigated rice fields.
While all of the traits and features contribute to the complexity and severity of the red rice problem in direct seeded rice culture, four are critical for the establishment and continuation of red rices as serious pests. The absence of any one of these traits would change the red rices to lesser, more easily managed weed pests, while the absence of any two of the critical traits would reduce them to the weedy status of volunteers of cultivated rice. The four critical traits, three of which are related to the ecology of the seed, are: the diversity and changeability of red rice populations resulting from natural crossing with cultivated varieties; early and heavy seed shattering; intense but variable seed dormancy; and superior seed and seedling vigor.
The natural crossing with cultivated varieties and resulting natural selection produces changes in populations of red rice that keep pace with changes in the type of varieties cultivated, e.g., development of semi-dwarfs. Early and heavy seed shattering insures that most of the red rice seeds are dispersed in the field rather than removed with the harvested grain. The intense but variable dormancy of the seeds insures that most of the red rice seeds do not germinate early and produce seedlings that succumb to adverse conditions during the interval between rice crops but remain dormant and germinate only when dormancy is released as conditions become favorable again for rice production. Dormant seeds that are deeply buried during land preparation for planting are deposited in the "soil seed bank" where dormancy and viability are maintained until the seeds are brought up to an emergence depth during cultivation in subsequent years. Red rice seeds deeply buried in the soil seed bank can retain their viability for many years and serve as a continuing source of infestation. The seeds of most red rice ecotypes germinate more rapidly over a wider range of conditions, emerge from greater depths and produce more vigorous seedlings than those of cultivated rice varieties.
Many strategies are employed to manage and control red rice infestations. The best strategy is to prevent infestations, but when the infestations are already present, rotation with another crop such as soybean to permit use of selective herbicides to kill the red rice and deplete the soil seed bank has been very effective. Presently, the main focus and interest is on herbicide resistant varieties, transgenic and non-transgenic. This control strategy, however, is being cautiously exploited due to the widely recognized and appreciated risk of development of herbicide resistant red rices.
A Winter Annual
Some of the most interesting and useful information on the ecology of seeds have been obtained from studies on annual, weedy and wild species of which the summer annual red rices are good examples. Summer annuals have to survive the freezing temperatures of the winter season that are lethal to emerged seedlings. But, what about the opposite season winter annuals? They germinate and emerge in the fall season, survive the winter in the vegetative stage, flower and produce seeds in the spring. Why don't the seeds germinate in late spring or summer like summer annuals? Many years ago when the phytohormone gibberellin was just being introduced we became unwittingly involved in examining these questions for Plantago aristata one of the rather obscure winter annual species of the weedy genus Plantago. We were surveying the effects of gibberellin on dormancy in a wide range of species and found that it released seed dormancy of P. aristata in darkness at 15 to 20°C and at 30°C in darkness or light. Our curiosity was aroused so we undertook other studies to try to determine why P. aristata was a winter annual. The seeds mature and shatter from the capsule in late May or early June. They are dormant and sensitive to temperature and light. They germinate in the light at temperatures around 20°C and somewhat cooler but not in darkness. At 30°C or warmer, which is the ambient temperature at the time of seed dehiscence, the seeds do not germinate in light or darkness. Thus, in nature they remain dormant until the temperature drops below about 25°C in late summer or early fall. Then, those that are at or near the soil surface and, thus, exposed to light germinate as soon as moisture is adequate, but those buried under debris or soil and excluded from light remain dormant and become the soil seed bank for P. aristata. We tried to induce germination in seeds 5 to 10 mm deep in soil at 30°C or warmer by spraying the seeds with a gibberellin solution before covering and drenching them with a gibberellin solution after covering with soil. Spraying gibberellin on the seeds before covering with soil induced about 20% germination while drenching them after covering produced only about 10% germination. The seedlings produced in soil plantings at temperatures of 30°C or warmer appeared to be weak and under stress but did not die during several weeks observations. Unfortunately, circumstances did not permit the continuation of the experiment for the remainder of the summer to determine the ultimate fate of the seedlings. It is unlikely, however, that they survived the 40°C soil temperatures in late summer and that we changed a winter annual to a summer and winter annual.
Some Final Thoughts
Seed ecology or ecophysiology is a fascinating but largely overlooked subject area, at least until now. I believe that in this era of biotechnology where most things seem possible the ecology of seeds and plants will finally receive the attention it deserves. In the past emphasis has been on altering, redesigning or locating ecological settings to fit crops and animals, but now emphasis seems to be changing to the "redesign" or alteration of crops and animals to fit specific ecological settings: saline soils, cooler temperatures, hotter temperatures, drier climates, wetter climates, soils with toxic levels of aluminum and other elements, and so on, perhaps even transforming a warm season crop into cool season crop. Anything now does seem possible!
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